I generally carry 9 round extended magazine as a backup for my Springfield Armory XDs 9mm. When I carried a S&W Shield, I carried the 8 round magazine as a backup, and when I carried a Glock 19, I kept a 17 round Glock 17 magazine as a backup.
The obvious thing here is that the extended magazines stick out of the grip of the pistol a bit more than the standard magazines, and in the case of the S&W Shield, and XDs 9mm, the extended magazines ship from the factory with plastic sleeves to fill in the area around the extended part of the magazine. These serve a couple purposes:
- Added Grip Area
- Preventing Over Insertion
If I’m in a gunfight, and I have to reload to a second magazine, I don’t really care too much about aesthetics. Adding extra grip is nice, but it’s not really anything I care about.
For the longest time I popped the sleeves off of my spare magazines so I didn’t have to worry about them sliding down the body of the mag and interfering with a reload.
I’ve changed my mind about this.
Months back, at one of the last Wake County Firearms Education and Training Center Action Pistol matches a friend was shooting his S&W Shield. He started with the 7 round magazine, and under the stress of the timer reloaded to an 8 round magazine (of which he had removed the sleeve for carry) and when he seated the magazine during a slide lock reload, the magazine slid past the magazine catch, and got stuck in the breech of the pistol. It took several minutes to get the magazine unstuck, something that would have put him out of the fight if it were a defensive scenario and not a game.
On my XDs 9mm, if my magazine slips past the mag catch on a slide-lock reload, the only thing that keeps the magazine from going all the way up into the breech is the ejector. I imagine if you were to seat the magazine hard enough you could damage the ejector.